Electro-optical system for gauging surface profile deviations

ABSTRACT

A method and system for gauging deviations of a surface of a test part from a preselected nominal surface profile is disclosed. One embodiment includes a support having a master surface that is substantially a matched or mating surface of the nominal surface profile of the test part and a thin layer of an attenuating medium such as a dye liquid between the master and test surfaces. In another embodiment, the interface between attenuation fluid and the air is used as a reference surface thereby eliminating the requirement of the master surface. In both embodiments, electromagnetic radiation is directed through the attenuating medium onto the surface of the test part to be gauged. An image sensor such as a camera is positioned to receive an image of the radiation reflected by the test part surface back through the attenuating medium, with the intensity of such radiation across the image varying as a function of the deviation of the test part surface from the nominal surface profile. The sensor output is digitized to form a series of digital signals indicative of the intensity of radiation associated with each location of the reflected image, and the digitized pixel signals are stored in digital electronic memory and/or displayed on a screen. Computer programming corrects the digitized intensity signals for sensor gain, bias and variations in part reflectivity, and presents a quantitative measurement of the deviations in test surface profile from the master surface profile over the entire surface being measured.

This application is a continuation-in-part of U.S. patent applicationSer. No. 07/770,885, filed on Oct. 4, 1991, now U.S. Pat. No. 5,289,267.

The present invention is generally related to a method and apparatus fordetermining the trueness of an object from a predetermined geometry.More specifically, this invention is related to a highly automated,electro-optical system for gauging deviations of a surface profile of atest part from a predetermined nominal profile geometry and a method forgauging deviations of such surface profiles. The method and apparatus ofthis invention are suitable for use with test parts having flat surfacesas well as contoured surfaces.

BACKGROUND OF THE INVENTION

It has heretofore been proposed to estimate flatness of a surface on atest part by visually observing reflection through a dye liquid film orlayer placed between the test surface and a flat master surface. Forexample, U.S. Pat. No. 2,695,544 discloses a system consisting of, inorder, a pane of glass, a dye layer, and the test part. Light isdirected through the pane of glass and into the dye layer. The operatorthen visually observes the light reflected by the test part surface backthrough the dye layer and the glass pane. Since the light energy isattenuated as a function of distance traveled through the dye layer,departure of the reflected light from uniform intensity across the imagegenerally indicates a corresponding departure of the test part surfacefrom flatness or parallelism with the surface of the glass pane. Thismethod is limited to a subjective and qualitative estimate of theflatness of the test object. This method is also limited by the visualacuity of the operator which will, of course, vary from operator tooperator. This method cannot account for differences in reflectivity ofthe test part across its surface or for differences in the illuminationor for other artifacts. This method is suitable for use only inrelatively less-demanding quality control applications where parts areeither accepted or rejected depending on their qualitative deviationfrom a prescribed geometry. This method is generally not suitable foruse in the operation, control, and/or modification of a manufacturingprocess wherein the parts are produced. This method is generally notuseful in quality control or other operations where it is necessary toquantitatively determine the deviations of the test part from aprescribed geometry.

It is desirable, therefore, to provide a system and a method for gaugingthe deviations of a test part from a predetermined nominal profilegeometry with enhanced and improved capabilities for precisequantitative measurement of surface deviations over the entire surfaceof the test part. It is also desirable to provide a system and a methodfor gauging the deviations of a test part from a predetermined nominalprofile geometry with improved measurement resolution that is adaptedfor use in conjunction with test parts having a wide variety ofgeometries and optical characteristics. It is also desirable to providesuch a system and method that includes facility for compensating againsteffects of background radiation, variations in illumination intensity,and variations in test part reflectivity. It is also desirable toprovide such a system and method that allows for improved control ofmeasurement resolution, and that is readily adapted for automation. Thepresent invention provides a system and method for gauging thedeviations of a test part from a predetermined nominal profile geometrywhich achieves these just described objectives and criteria.

SUMMARY OF THE INVENTION

The present system for gauging deviations of a surface on a test partfrom a preselected nominal surface geometry includes a support that isessentially transparent to the electromagnetic radiation used and has amaster surface that is substantially a matched or mated surface to thepreselected nominal surface geometry of the test part. The terms"matched surface" or "mated surface" as employed in the presentapplication mean that the master surface 18 essentially contains thecomplement image of the prescribed nominal surface geometry which is thedesired profile of the test part such that, when the master surface andthe test part are brought into adjacent opposition as shown in FIG. 2,the separation between the master surface and the test part will beessentially uniform across the surfaces. For example, if the nominalsurface geometry of the test part is flat, the master surface of thesupport is likewise flat. If the nominal surface of the test part is ofconvex curved shape, the master surface of the support is ofcomplementary concave curved shape. An essentially non-scattering orlow-scattering attenuating medium is placed on the master surfacebetween the test surface and the master surface, with the test partbeing supportable on the master surface with the surface of the testpart opposed to the master surface. The attenuating medium may be a dyefluid or any appropriate medium (fluid, powder, or gas) providing thatthe medium attenuates the electromagnetic radiation with minimalscattering, and that the medium freely flows into and substantiallyfills the voids to be gauged between the master surface and the testsurface. The test part may be supported by the attenuating fluid (suchas a dye fluid) itself or, preferably, by support shims or othermechanical devices to help ensure nominally uniform spacing between thetest and master surfaces. The attenuating medium should substantiallyfill the spaces to be gauged between the master surface and the testpart surface.

A source of electromagnetic radiation is positioned to direct suchradiation through the support and into the dye fluid. The radiation,preferably in the visible light spectrum, is directed into the dye fluidthrough the support and master surface. An image recording system ispositioned to receive the radiation reflected off the test part and backthrough the dye fluid and thereby record an image of electromagneticradiation transmitted from the dye fluid. The intensity of the radiationacross the image will vary as a function of the deviations of the testpart surface from the nominal geometry. The image received by the imagerecording system is digitized to form a series of digital signalsindicative of intensity of radiation received at sequential pictureelements or pixels of the image. An electronic memory receives, stores,and manipulates such digital signals as necessary.

The present invention uses the attenuation of electromagnetic radiationpassing through a medium to determine and measure the deviations of atest part from that of the nominal and desired profile geometry. In thecase of visible light passing through a dye layer, the light energy isattenuated exponentially as a function of the distance traveled throughthe dye layer. The measured deviations of the reflected light fromuniform intensity (i.e., the variations in intensity of the reflectedlight across the surface) can be used to calculate the relativedeviations of the test part surface from the master surface. The use ofsuitable calibration standards allows the determination of the absolutedeviations of the test part surface from the master surface.

Implementation of the present invention provides a two-dimensional imageof the test part surface profile in a form suitable for digitalmanipulation, processing, and analysis purposes within a computer systemusing appropriate software techniques. The digital image of the testpart surface profile or digital data corresponding to the test partsurface profile may be readily displayed or plotted in the form of atwo-dimensional image illustrating the deviation profile or, with propercomputer enhancement, displayed or plotted in the form of athree-dimensional image illustrating the deviation profile. Orcross-sectional views of the deviation profile can readily be obtainedthrough critical surface areas of the test part. The digital image mayalso be employed using conventional manufacturing process controltechniques to automatically correct a part production process to reduceor eliminate profile deviations in the test part or to correct forvariations over time in the part production process due, for example, towear or variations in the cutting process or tooling members. Digitalprocessing and software techniques may be employed to correct fornon-uniform illumination of the test part, distortion and/or gainvariations in the imaging camera, non-uniformities in surfacereflectivity of the test part, variations in dye characteristics acrossthe image, and other artifacts.

The present invention can be utilized for measuring deviations ofsurface profiles from a reference profile master under a variety ofconditions. For each condition, a preferred wavelength and attenuatingmedium can be selected that is based on the costs or otherconsiderations (e.g., desired resolution, tolerances, safetyconsiderations, and the like) relating to the imaging and digitallyrecording the reflected electromagnetic wave at different wavelengths.For example, to measure deviations on the order of thousandths of aninch, optical frequencies in the visible region and a dye fluid are thepresently preferred embodiment of the invention because suitableinexpensive illumination systems and digitizing cameras exist for use inthis embodiment. However, if it is desired to measure larger sizedsurface deviations, microwave radiation might be used as theilluminating radiation with a resistive dielectric fluid as theattenuating medium.

As one skilled in the art will realize, the manufacture of mastersupport and its master surface will become more expensive andtechnically difficult as the dimensions of the test part increases. Itwould be desirable, therefore, to provide a technique and apparatuswherein the master support and its master surface is not required sothat the inventive methods of the present invention can be more easilyapplied to very large parts such as, for example, automobile doors andthe like. The present invention provides such a system. The test part tobe evaluated is placed in a container of the attenuating fluid (e.g.,dyed fluid) whereby the test part--or at least the surface to beevaluated--is completely submerged in the attenuating fluid. Theinterface of the attenuating fluid and air is used as the referencesurface, thereby eliminating the master support and its master surface.For test parts with a depression, valley, or basin that can receive andhold the attenuation fluid (i.e., a fluid receptacle), the fluidreceptacle can be filled with attenuation fluid and the contour of thetest part surface within the receptacle can be determined using theattenuation fluid/air interface as the reference surface.

One object of the present invention is to provide a system for gaugingdeviations of a surface on a test part from a preselected nominalsurface geometry using electromagnetic radiation, said systemcomprising:

(1) a master surface that is a substantially matched surface of thepreselected nominal surface geometry and that is essentially transparentto the electromagnetic radiation;

(2) an attenuating medium on the master surface with the test part beingsupportable on the master surface with the surface of the test partopposed to the master surface such that the attenuating mediumsubstantially fills all the space to be gauged between the mastersurface and the test part surface;

(3) a source of electromagnetic radiation positioned to direct suchradiation through the master surface and into the attenuating medium;

(4) an image sensor positioned to receive an image of electromagneticradiation transmitted from the attenuating medium;

(5) a digitizer for converting the image from the image sensor intodigital signals indicative of the intensity of the radiation at thelocations of the image; and

(6) digital electronic storage coupled to the digitizer for receivingand storing the digital signals;

whereby the intensity of the transmitted radiation varies across theimage as a function of the deviation of the test part surface from thepreselected nominal surface geometry.

Another object of the present invention is to provide a method ofgauging deviations of a surface on a test part from a preselectednominal surface geometry using electromagnetic radiation and a mastersurface that is essentially transparent to the electromagnetic radiationand that contains a substantially matched surface of the preselectednominal surface geometry, said method comprising:

(1) forming an attenuating medium layer on the master surface;

(2) placing the test part on the attenuating medium layer such that thesurface of the test part is opposed to the corresponding matched surfaceof the master surface and such that substantially all spaces to begauged between the test part surface and the master surface areessentially filled with attenuating medium;

(3) passing electromagnetic radiation into the attenuating medium in thedirection of the test part surface;

(4) collecting the electromagnetic radiation which is transmitted fromthe attenuating medium to form an image of the transmittedelectromagnetic radiation;

(5) digitizing the image of the transmitted electromagnetic radiation;

(6) storing the digitized image in an electronic storage deviceassociated with a computer; and

(7) determining the deviations of the surface of the test part from apreselected nominal surface geometry using computer software techniques;

whereby the intensity of the transmitted electromagnetic radiationvaries across the image as a function of the deviation of the test partsurface from the preselected nominal surface geometry.

Another object of the present system is to provide a system for gaugingdeviations of a surface on a test part from a preselected nominalsurface geometry using electromagnetic radiation, said systemcomprising:

(1) a container holding the test part;

(2) an attenuating fluid within the container holding the test part suchthat the attenuating fluid substantially covers the test part surface tobe gauged and such that an attenuation fluid/air interface is formedwhich is suitable for use as a reference surface;

(3) a source of electromagnetic radiation positioned to direct suchradiation through the reference surface and attenuating fluid and uponthe test part surface to be gauged;

(4) an image sensor positioned to receive the electromagnetic radiationwhich is reflected from the test part surface to be gauged and whichpasses back through the attenuating fluid and across the referencesurface, whereby an image of the test part surface to be gauged isformed;

(5) a digitizer for converting the image from the image sensor intodigital signals indicative of the intensity of the radiation atlocations of the image; and

(6) digital electronic storage coupled to the digitizer for receivingand storing the digital signals;

whereby the intensity of the transmitted radiation varies across theimage as a function of the distance of the test part surface to begauged from the reference surface.

Still another object of the present invention is to provide a method ofgauging deviations of a surface on a test part from a preselectednominal surface geometry using electromagnetic radiation, said methodcomprising:

(1) placing the test part in a container holding an attenuating fluidsuch that the surface of the test part to be gauged is submerged in theattenuation fluid;

(2) allowing the attenuation fluid to settle such that the interfacebetween the attenuation fluid and the air is suitable for use as areference surface;

(3) passing electromagnetic radiation through the reference surface andinto the attenuating fluid in the direction of the test part surface;

(4) collecting the electromagnetic radiation which is reflected from thetest part surface and transmitted from the attenuating fluid to form animage of the transmitted electromagnetic radiation;

(5) digitizing the image of the transmitted electromagnetic radiation;

(6) storing the digitized image in an electronic storage deviceassociated with a computer; and

(7) determining the deviations of the surface of the test part from apreselected nominal surface geometry using computer software techniques;

whereby the intensity of the transmitted electromagnetic radiationvaries across the image as a function of the distance of the test partsurface from the reference surface.

Still another object of the present invention is to provide a system forgauging deviations of a surface on a test part from a preselectednominal surface geometry using electromagnetic radiation where the testpart surface to be gauged forms a fluid receptacle, said systemcomprising:

(1) an attenuating fluid filling the fluid receptacle such that anattenuation fluid/air interface is formed which is suitable for use as areference surface;

(2) a source of electromagnetic radiation positioned to direct suchradiation through the reference surface and attenuating fluid and uponthe test part surface to be gauged;

(3) an image sensor positioned to receive the electromagnetic radiationwhich is reflected from the test part surface to be gauged and whichpasses back through the attenuating fluid and across the referencesurface, whereby an image of the test part surface to be gauged isformed;

(4) a digitizer for converting the image from the image sensor intodigital signals indicative of the intensity of the radiation atlocations of the image; and

(5) digital electronic storage coupled to the digitizer for receivingand storing the digital signals;

whereby the intensity of the transmitted radiation varies across theimage as a function of the distance of the test part surface to begauged from the reference surface.

Still another object of the present invention is to provide a method ofgauging deviations of a surface on a test part from a preselectednominal surface geometry using electromagnetic radiation where the testpart surface to be gauged forms a fluid receptacle, said methodcomprising:

(1) filling the fluid receptacle with an attenuation fluid;

(2) allowing the attenuation fluid to settle such that the interfacebetween the attenuation fluid and the air is suitable for use as areference surface;

(3) passing electromagnetic radiation through the reference surface andinto the attenuating fluid in the direction of the test part surface;

(4) collecting the electromagnetic radiation which is reflected from thetest part surface and transmitted from the attenuating fluid to form animage of the transmitted electromagnetic radiation;

(5) digitizing the image of the transmitted electromagnetic radiation;

(6) storing the digitized image in an electronic storage deviceassociated with a computer; and

(7) determining the deviations of the surface of the test part from apreselected nominal surface geometry using computer software techniques;

whereby the intensity of the transmitted electromagnetic radiationvaries across the image as a function of the distance of the test partsurface from the reference surface.

These and other objects of the present invention will be explained inmore detail in the following description of the preferred embodiments ofthe invention with reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an inspection station using the gauging system ofthe invention.

FIG. 2 is a schematic diagram of a gauging system in accordance with apresently preferred embodiment of the invention.

FIG. 3 is a schematic diagram of the test part surface profile imageviewed by the camera in FIG. 2.

FIG. 4 is a fragmentary schematic diagram of a modified embodiment ofthe invention.

FIG. 5 is a fragmentary schematic diagram of a technique for calibratingthe embodiment of the invention illustrated in FIG. 2.

FIG. 6 is a fragmentary schematic diagram of a modified embodiment ofthe invention wherein the reference surface is the fluid-air interface.

FIG. 7 is a fragmentary schematic diagram of a modified embodiment ofthe invention wherein the reference surface is the fluid-air interfaceand the fluid is contained within a depression or hollow area in thetest part.

FIG. 8 is a computer-generated image of a standardized test partgenerated by the gauging system of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 generally illustrates an inspection station 100 using the gaugingsystem of this invention. The gauging system is shown in more detail andin different embodiments in FIGS. 2 through 5. FIG. 1 shows aninspection station 100 consisting of a test chamber enclosure 102, anelectromagnetic source and image sensor compartment 104, and theassociated computer work station 106. The computer work station 106isshown with a display screen, input device (i.e., a keyboard), and acabinet to contain the associated computer hardware, memory, andinterface devices. The test chamber enclosure 102 contains the test part14 and test surface 12 which is to be gauged for deviations from apreselected nominal surface geometry. Using fixture 108, the test part14 is lowered into or placed in an attenuating medium 20 using fixture108 such that there is a thin film 24 of the attenuating medium 20between the test part surface 12 and the master surface 18 of the mastersupport 16. The master support 16 must be transparent to theelectromagnetic radiation used. The master support 16, in this case anoptical flat, provides the interface between the test chamber enclosure102 and the electromagnetic radiation source and image sensorcompartment 104. Compartment 104 contains the electromagnetic source 32and the image sensor 34. In FIG. 1, two electromagnetic radiationsources 32 are used. As shown in FIG. 2, only one electromagneticradiation source can be used; or, if desired, more than twoelectromagnetic radiation sources can also be used. As explained in moredetail below, electromagnetic radiation from the electromagneticradiation source 32 is directed through the master support 16 and itsmaster surface 18, into the attenuation film 24, onto test part surface12, and then back through the attenuating film 24 and master support 16to the image sensor 34. The distance the electromagnetic radiationtravels through the attenuation film 24 is generally equal to twice thedistance between the surfaces 12 and 18 at any given point on thesurface 12. By appropriate manipulation, the electromagnetic imagereceived at image sensor 34 is converted into digital signals suitablefor computer manipulation. Using computer and suitable softwaretechniques via the computer work station 106, the deviations of the testpart surface 12 from a preselected nominal geometry can be determinedand displayed as detailed below.

As one skilled in the art will realize, the components in FIG. 1 can bereoriented in various ways. For example, the test chamber compartment102 and the electromagnetic radiation source and image sensorcompartment 104 can be rotated 180 degrees relative to each other suchthat the test part surface 12 would be located below the electromagneticradiation source 32 and image sensor 34. Or the compartments 102 and 104can be arranged side-by-side with the master support 16 beingessentially a vertical interface between the two compartments (i.e., thetest part 14 is located on one side of the master support 16 and theelectromagnetic radiation source 32 and the image sensor 34 on the otherside). This side-by-side arrangement may be especially useful toeliminate or minimize air bubbles which might otherwise be trappedbetween the surfaces 12 and 18.

FIG. 2 illustrates a system 10 in accordance with an exemplary butpresently preferred embodiment of the invention for gauging or measuringdeviations of the surface 12 on a test part 14 from a preselectednominal surface geometry, in this case a flat geometry. System 10includes a support 16 having a master surface 18 that is manufactured tobe a substantially exact mating or matched surface of the nominalsurface geometry of test surface 12 (i.e., a flat surface). As notedabove, the terms "matched surface" or "mated surface" as employed in thepresent application mean that the master surface 18 essentially containsthe complement image of the prescribed nominal surface geometry which isthe desired profile of the test part such that, when the master surfaceand the test part are brought into position as shown in FIG. 2, theseparation between the master surface and the test part will beessentially uniform across the surfaces. In other words, if theprescribed nominal geometry of the test part contains, for example, abulge in the shape of a pyramid, the master surface will contain acorresponding depression in the shape of a pyramid. Or if the prescribednominal geometry of the test part is flat, the master surface will beflat. It is not necessary, however, that the master surface conformexactly in every detail to the prescribed nominal surface geometry ofthe test part. Variations between the master surface and the prescribednominal surface geometry can be corrected or accounted for usingsoftware techniques.

An attenuation medium 20, preferably a dye liquid, is carried on mastersurface 18 of support 16. Test part 14 rests on a plurality of shims orspacers 22 that separate master surface 18 from test surface 12 by anominal distance corresponding to thickness of the shims. It isgenerally preferred that the shims 22 have the same thickness. In somecases, however, it may be preferred that shims of different thicknessare used. Dye liquid 20 thus forms a fluid film or layer 24 betweensurfaces 12 and 18 and fills the voids and depressions 26, 28, and 30 insurface 12 of test part 14. Generally, the distance between the twosurfaces 12 and 18 (i.e., the nominal thickness of the attenuationmedium 20) should be minimized. Generally, a separation distance ofabout 0.01 to 0.05 inches will be satisfactory. Separations of the twosurfaces 12 and 18 substantially greater than or less than these limitsmay, however, be employed.

The attenuation medium 20 should fill the voids, depressions, grooves,and other features to be gauged between the two surfaces 12 and 18. Insome instances, they may be imperfections or designed features of thetest part which are located in areas of the part that are not critical.If such areas are not to be examined, it is, of course, not necessarythat such features be filled with the attenuation medium 20. In manycases, it may even be preferred that such areas (i.e., the non-interestareas) be blocked or masked out to simplify analysis. By masking theseareas, the operator can concentrate on the critical areas of interest.Masking can be done with suitable software techniques.

In some instances, air bubbles may become entrapped between the surfaces12 and 18, especially in pockets 26, depressions 28, or cracks 30 insurface 12 of test part 14. Although it is generally preferred that suchair bubbles be minimized, it is not necessary that they be completelyeliminated. Such air bubbles can be minimized by appropriate ventingchannels so that the bubbles can escape, careful orientation of the partas it is placed in the attenuation medium so that entrapment of suchbubbles is minimized, vibration of the part or system so that thebubbles can escape, the use of degassed solvents, and the like. As notedabove, air bubbles can also be minimized by orienting the compartments102 and 104 in a side-by-side arrangement with the master support 16 inan essentially vertical orientation.

An electromagnetic radiation source 32 is positioned beneath support 16and directs light energy through support 16 into film 24 of dye liquid20 between surfaces 16 and 18. Support 16 is substantially transparentto such light energy from source 32. Light energy from source 32 istherefore incident on surface 12 of test part 14 through support 16 andfilm 24, and is reflected by the test part surface back through the dyeliquid film 24 and the support 16. An image sensor 34 (e.g., a camera)is positioned beneath support 16 adjacent to light source 32, and isoriented with respect to support 16 so as to receive the attenuatedreflections from the surface 12. The image sensor 34 must be responsivein a predictable manner to the electromagnetic radiation used, mustprovide the desired resolution, and must be capable of generating datawhich can be digitized. Suitable image sensors include vidicon cameras,charge coupled devices (CCDs), image array sensors, and the like. Asshown in FIG. 3, image sensor 34 preferably comprises a CCD sensor 36having a matrix of image sensing elements 38 in a row-and-column array.Each element 38 thus receives a corresponding portion or pixel of theoverall image of test part surface 12. Cameras with variable focallengths or zoom lenses are often preferred because they allow theresolution of the system to be varied relatively simply. In someinstances, however, cameras with fixed focal lengths may be preferred.For example, a gauging system dedicated to a manufacturing processproducing a single part might not need variable resolution capabilities.Cameras capable of interfacing with the computer and, therefore, beingcontrolled by the computer are especially preferred.

Camera 34 is connected through suitable digitizing electronics 40 to acomputer 42 that includes digital memory 44 for receiving and storingthe digitized pixel signals from camera 34. Image data is thus stored asnumeric data indicating the intensity of the electromagnetic radiationreceived for each pixel in the matrix of pixels. Computer 42 alsoincludes a screen 46 for displaying to an operator the stored image oftest part surface 12. The stored image or data can be displayed, withsuitable computer manipulation or enhancement, as shades of gray or invarious colors to illustrate deviations from the prescribed nominalgeometry. As shown in FIG. 8, the digital data can also be printed orplotted as desired using suitable computer-graphic techniques. Thedigital data (in either its raw or manipulated forms) can be storedindefinitely to allow for long-term quality control analysis. Such datamight be useful, for example, to study failures of critical componentswhere the actual failed components are not readily available (e.g.,satellite malfunctions) or to perform long-term statistical analysis offailure or reject rates to pinpoint and correct manufacturing problems.

In operation, light energy from source 32 is incident on surface 12through support 16 and dye film 24, and is reflected by surface 12 backthrough the dye film 24 and support 16 to camera 34. Such light energyis attenuated during two passes through dye film 24 as a function ofdistance traveled through the dye film. Thus, if the film is of uniformthickness, meaning that surfaces 12 and 18 are parallel to each otherthroughout the image area, the image of test part surface 12 will be ofuniform intensity (assuming uniform reflectivity across the surface 12).On the other hand, any pockets 26, depressions 28, or cracks 30 insurface 12 of test part 14 will necessarily increase the distance thatthe light travels through the dye film, resulting in darker sections26a, 28a, and 30a in the image 12a of the test part surface asillustrated in FIGS. 2 and 3. In the same way, any outward protrusionsin surface 12 of test part 14 (not illustrated in the drawings) willresult in a correspondingly reduced distance of light travel through dyefilm 24 and correspondingly lighter areas of the test surface image.Intensity variations of the image portions 26a, 28a, and 30a aredirectly related to depth of the corresponding surface irregularities,and the area of each image portion corresponds to the area of thecorresponding depression in the overall surface. By accounting forsystem geometry and illumination variations, the transmission propertiesof support 16 and attenuation properties of dye fluid 20, thetwo-dimensional reflected image of test part surface 12 is convertedwithin computer 42 to a digitized two-dimensional map 12a of test partsurface contour. Map regions of interest may be selected and magnifiedby the operator. By suitably calibrating the system in ways to bedescribed, precise quantitative measurements of surface profiledeviations can be obtained for analysis and/or control purposes.

FIG. 4 illustrates a modified embodiment of the invention for gaugingthe profile of a test part 47 having a curved test surface 48. Themaster surface 50 of support 51 is either machined as a matched or matedsurface 48 of the nominal desired geometry into a glass support using,for example, a diamond lathe or is cast into a slab of suitabletransparent material. In this connection, it will be appreciated that,although master surface 50 (FIG. 4) or 18 (FIG. 2) is employed as areference surface for gauging purposes, the master surface need not bean exact replica of the nominal test part surface geometry. Smalldeviations in profile between the nominal surface geometry and themaster reference surface can be accommodated by suitably calibratingcomputer 42 with a dye liquid between the master surface and the testpart surface (i.e., surfaces 18 and 12 in FIG. 2 and surfaces 50 and 48in FIG. 4) using a test part predetermined to possess a surface ofdesired nominal contour. Such a test part (i.e., one known to have orspecifically manufactured to have the predetermined nominal surfacegeometry) may be retained as a "standard" for routine calibrationpurposes. If the amount of light reflected from all points on thestandard test object surface during this calibration operation isuniform throughout, the thickness of the dye film is uniform and nocorrections need to be made. On the other hand, any deviations betweenthe master reference surface and the opposing surface of the standardpart will result in a corresponding variation in intensity at one ormore pixels of the reflected surface image. By measuring and storingthese pixel signals at all points on the surface image, computer 42effectively captures the correct profile of the standard part withrespect to each opposing or corresponding point on the master surface.The information so obtained can then be employed to offset or bias thecorresponding pixel signal or signals during operation of the system soas to accommodate any deviations in the master reference surface.

It cannot always be assumed that the surface of the test part reflectsthe test illumination uniformly along the entire test part surface.Variations in machining, stains, or material composition can cause thereflectance of the test part surface to vary. The system in accordancewith the present invention can, however, be calibrated to accommodatesuch variations in test part surface reflectivity. In one approach, thereflectivity of the test surface is measured first with clear fluid andthen with the same fluid containing a dye. These measurements can thenbe used to calibrate the varying reflectivity of the test part surfaceand eliminate the effects caused by the differences in reflectivity.Digitization of the data as provided in the present invention allowssuch corrections to be made in a straight forward manner using softwaretechniques.

In a second approach as illustrated in FIG. 2, the effects of surfacereflectivity are removed by making measurements at two separate averagewavelengths λ₁ and λ₂. An optical filter 52 is used to select thewavelength recorded by the camera. The filter is coupled to a suitabletranslation device 54 controlled by computer 42 for selectivelytranslating filter 52 into and out of the path of the test part surfaceimage incident on camera 34. A first image of the test part surface isobtained with filter 52 removed from the image path, as illustrated inFIG. 2. This first image is taken at an averaged spectral wavelength λ₁to which the dye has a spectrally averaged absorption coefficient α₁. Asecond image is obtained with filter 52 intersecting the image path.This second image is taken at an averaged spectral wavelength λ₂ towhich the dye has a spectrally averaged absorption coefficient α₂. Forthe image obtained with wavelength λ₁, the measured intensity of thereturned or reflected light I_(m1) at a given pixel location isdescribed by the equation

    I.sub.m1 =I.sub.i1 exp(-2α.sub.1 d)R.sub.1

here I_(i1) is the effective incident intensity at that pixel location,d is the thickness of the dye layer at that pixel location, and R₁ isthe reflectivity of the surface at that pixel location. Similarly, themeasured intensity of the returned or reflected light I_(m2) forwavelength λ₂ at that same pixel location is given by the equation

    I.sub.ms =I.sub.i2 exp(-2α.sub.2 d)R.sub.2

where I_(i2) is the effective incident intensity at that pixel location,d is the thickness of the dye layer at that pixel location, and R₂ isthe reflectivity of the surface at that pixel location. Assuming thatthe surface reflectivity is independent of wavelength, which is areasonably good approximation for most metals, R₁ equals R₂ in the twoabove equations for each pixel location. The ratio of the measuredintensities at the two wavelengths is thus given by the followingequation

    I.sub.m2 /I.sub.m1 =(I.sub.i2 /I.sub.i1)exp{-2(α.sub.2 -α.sub.1)d}

which no longer involves the reflectivities R₁ and R₂ of the surface. Inthis equation, all parameters are known except the ratio I_(i2) /I_(i1)and the distance d to be determined. The ratio I_(i2) /I_(i1) can bedetermined using a calibration groove or line of know depth (i.e., ashim 22 could contain a groove of known depth). (Alternatively, aphotodiode or other light measuring device can be used to directlymeasure the intensity of the incident radiation at each wavelength and,therefore, determine the unknown ratio I_(i2) /I_(i1) in the aboveequation.) Solving the above equation for d yields the followingequation

    d=1n{(I.sub.m1 I.sub.i2)/(I.sub.m2 I.sub.i1)}/{2(α.sub.2 -α.sub.1)}

for each pixel location, which is independent of the reflectivity of thesurface. This method for correcting for differences in reflectivity ofthe test surface is ideally suited computer manipulation of thedigitized data.

As noted, this just described method for correcting for differences inreflectivity requires making measurements at two separate averagedwavelengths λ₁ and λ₂. In the above described procedure, the filter 52was moved in and out of the image path between the surface of interestand the camera 34. Other procedures could be used to obtain the data atthe two wavelengths. For example, two different filters with differentspectral characteristics could be used. Or the filter 52 or differentfilters could be placed between the light source 32 and the surface ofinterest. The actual method by which the measurements at the twoseparate averaged wavelengths λ₁ and λ₂ are obtained is not critical.

Provision of the test part surface image in digital form suitable forstorage and processing in accordance with the present invention readilyaccommodates calibration. For example, gain associated with each pixelof the surface image can be obtained and employed during operation in amanner analogous to that disclosed in U.S. Pat. No. 4,960,999 which isassigned to the same assignee as the present application and which ishereby incorporated by reference. Because the test part surface may notbe uniformly illuminated by the light source 32, or the response of thecamera elements may be spatially non-uniform, the system of the presentinvention preferably includes the capability of correcting fornon-uniform illumination and/or detector response. Specifically and asillustrated in FIG. 5, if during a measurement the illumination geometrydoes not change and the strength of the illumination is held constant,spacial variations in illumination uniformity are accommodated byplacing an object 56 having a surface 58 of known uniform reflectance inplace of the test object on master surface 18 without the presence ofattenuating fluid. The reflected image can then be measured and used tocreate a two-dimensional map of correction data to normalize thereflected image pixels during a test operation with the fluid in place.This two-dimensional map of correction data need only be reobtained ifsystem geometry or detector characteristics change. For a system inwhich the test part surface occupies a large portion of the field ofview of the camera, the light path through the dye film may not beperpendicular to the master and test surfaces across the entire image.However, such non-uniform optical path lengths can readily beaccommodated through calibration techniques and generation of correctionmaps in a manner similar to that described immediately above as long asthe size of the test part and the physical positioning of the lightsource, test part, and camera remain constant.

As one skilled in the art will realize, the manufacture of mastersupport and its master surface will become more expensive andtechnically difficult as the dimensions of the test part increases. Itwould be desirable, therefore, to provide a technique and apparatuswherein the master support and its master surface is not required sothat the inventive methods of the present invention can be more easilyapplied to very large parts such as, for example, automobile doors andthe like. This can be accomplished by placing the part to be evaluatedin a container of the attenuating fluid (e.g., dyed fluid) whereby thepart is completely submerged in the attenuating fluid. FIG. 6illustrates the gauging apparatus of the present invention where thetest part is submerged within the attenuating fluid and the interface ofthe attenuating fluid and air is used as the reference surface. As shownin FIG. 6, the test part 114 is placed or submerged in the attenuatingfluid 120 contained in fluid tank 170. Electromagnetic radiation fromelectromagnetic radiation sources 132 is passed through the attenuatingfluid/air interface 172, through the attenuating fluid, striking thetest part surface 112, and then directed to the image sensor 134 (e.g.,a camera). The attenuating fluid/air interface acts as, and replaces,the reference or master surface 18 of FIGS. 1, 2, and 5. Theelectromagnetic radiation is attenuated as it passes through theattenuating fluid and the amount of attenuation is dependent on thedistance traveled. By measuring the attenuated electromagnetic radiationreflected from the surface 112, the deviations from a known geometry canbe determined using the same procedures as described in thisspecification. In other words, the data from the image sensor 134 isdigitized and then sent to, and manipulated by, a computer (not shown)in the same manner as described above. Various filters (also not shown)can also be used in the same manner as described above.

In operation, test part 114 is placed in the tank 170 of attenuatingfluid 120. Once the attenuating fluid surface 172 has settled, the flatsurface (i.e., the attenuating fluid/air interface) is suitable for useas a reference or master surface. To decrease the time required for thesurface 172 to stabilize and to minimize vibrations of the surface 172during operation, baffles 174 or other damping devices can be used. Forexample, the tank 170 could be mounted on a vibration damping pad orsupport (not shown). Or a thin film of a higher viscosity fluid could beplaced on the fluid surface. For example, if water is used as theattenuation fluid 120, a thin layer of oil could be applied to the watersurface to reduce waves or ripples. Such a thin film may also serve asan antireflective coating. Examples of suitable attenuating fluidinclude water, oils, silicone fluids, organic liquids, and the like.Generally fluids of relatively low viscosity and relatively lowvolatility are preferred. As one skilled in the art will realize, theresolution of the present system using the attenuating fluid/airinterface as the reference surface will be dependent, in large part, onthe stability of that interface. Thus, measures that enhance thatstability, including, for example, minimization of vibrations andreduction of evaporation from the surface, will increase the resolutionobtainable and are, therefore, preferred. For attenuating fluids thatare relatively volatile, the evaporation can be reduced, and theresolution increased, by applying a thin film of a non-volatile,non-miscible, low density liquid or fluid to the surface of theattenuating fluid. The attenuating effect of such a surface film orlayer can be determined during calibration and then accounted for duringroutine operations.

Once the surface 172 has settled, electromagnetic radiation from sourceor sources 132 is directed towards the part surface 112 to be evaluated.The electromagnetic radiation passes through the attenuating fluid 120,is reflected off the surface 112, passes again through the attenuatingfluid 120, and is collected at the image sensor 134. By measuring theattenuation of the electromagnetic radiation across the surface 112, thesurface profile can be determined. If the desired surface profile isflat, the attenuation fluid/air interface can be used as the directreference surface and deviations from a flat planar surface can bemeasured directly. If the desired surface geometry deviates from planar,a calibrated or reference test part can be used to calibrate theinstrument. The surface profile of the calibrated test part can bestored in the computer and then compared to the data generated from testpart 114 to determine deviations from the desired surface profile.

FIG. 7 illustrates the use of the attenuating fluid/air interface as thereference surface where the test part has a depression, valley, or basinthat can receive and hold the attenuation fluid (i.e., a fluidreceptacle). As shown in FIG. 7, a depression 180 of a test part 114 isfilled with attenuation fluid 120. The attenuation fluid/air interface172 acts as the reference or master surface in the same manner asdescribed for FIG. 6 above. In this manner, the surface profile 112 ofthe depression 180 can be determined. As one skilled in the art willrealize, this modified method can generally only be used to evaluatesurfaces within depressions 180 which can contain the attenuating fluid120. Thus, this modified method is not suitable for evaluation of theflat portions 182 of the test part or other portions that could not holdand contain the attenuating fluid. This method could be used toadvantage for large parts where the critical dimensions or profiles ofinterest are contained, or predominantly contained, within depressionsor hollowed-out areas in the test part.

As one skilled in the art will realize, other gases, including a vacuum,could be used above the attenuating fluid to form the attenuatingfluid/air interface for use as a reference surface. Generally, ambientair is preferred due mainly to ease of operation and avoidance of gascontainment and handling systems. Other gases can, of course, be used,including inert gases such as carbon dioxide, nitrogen, argon, and thelike. Mixtures of gases can also be used. As used in this specificationin regard to the attenuating fluid/air interface, the term "air" isintended to be a generic term including such gases.

FIG. 8 illustrates the type of results that the present system cangenerate. A test part was prepared by machining a series of parallelgrooves of varying depth in a metal block. In addition, on a portion ofthe flat surface between two grooves, a shallow, long depression was cutto simulate a surface defect. Using visible light with a dye fluid(i.e., india ink) as the attenuation medium, the image of the surface ofthis test part was generated using the present invention. A portion ofthe resulting image is shown in FIG. 8 where the x-axis is labeled 94,the y-axis is labeled 96, and the z-axis is labeled 98. The units forall three axes are given in inches. The five grooves cut in the testsurface can clearly be seen: groove 82 is 0.0001 inches deep; groove 84is 0.0002 inches deep; groove 86 is 0.0003 inches deep; groove 88 is0.0004 inches deep; and groove 90 is 0.0005 inches deep. The simulatedsurface defect 92 is seen between grooves 82 and 84. As shown in FIG. 8,depressions as small as 0.0001 inches can readily be observed andmeasured using the present invention.

For larger parts, it may be desirable to correct the intensity datareceived at the image sensor for the increase in optical depth towardthe edge of the camera's optical field. The corrected intensity I^(c)(x,y) at a point (x,y) can be found from the following equation:

    I.sup.c (x,y)=I(x,y)[cos(θ)]

where I(x,y) is the uncorrected intensity at point (x,y) and θ is theangle between the camera's optical axis and the light ray from thecamera to point (x,y). When the angle θ is small this correction is alsosmall and can, therefore, be disregarded. Thus, with relatively smallparts, which can fit into a narrow portion of the camera's opticalfield, this correction can usually be omitted except where the highestdegree of accuracy is needed. Even for larger parts, the camera can bemoved relative to the part's surface and multiple images of the surfacetaken such that all surfaces of interest are contained and recordedwithin a narrow portion of the camera's optical field.

As noted above, the various corrections and calibrations can be carriedout using conventional software techniques. Such software techniques arewell know in the art and need not be specified here in great detail.Generally, although other general procedures could be used, thesesoftware techniques involve storing intensity data for each location(x,y) or pixel location in a separate computer register. The correctionsor other manipulations would simply involve multiplying the contents ofthe appropriate register by a suitable factor (i.e., cos(θ) of the aboveequation) or adding or subtracting the appropriate intensity amount forthe calibration procedures employed to the contents of the appropriateregister. Not all corrections or calibrations described herein will beappropriate or need to be made for every part. After all desiredcorrections or manipulations are made, the intensity data can be plottedusing suitable computer-graphics techniques. Such graphics software isavailable commercially. For example, suitable graphics software can beobtained from Research Systems Inc. of Boulder, Colo. or from ImagingTechnology Inc. of Woburn, Mass. Other graphics software packages canalso be used.

The use of electromagnetic energy in the visible spectrum for bothillumination of and reflection from the test part surface is generallypreferred. However, electromagnetic radiation from the x-ray region tothe microwave region may be employed and may, in some instances, bepreferred. An attenuating medium using a liquid attenuating dye fluid isgenerally preferred for use at visible wavelengths. Normally, such anattenuation medium would consists of a dye dissolved in a solvent.Suitable dyes include, for example, india ink, FD&C Blue No. 1, D&CYellow No. 2, D&C Green No. 6, trans-β-carotene, and the like. Suitablesolvents include, for example, water and organic solvents such asalcohols (e.g., methanol, ethanol, tert-butyl alcohol, amyl alcohol, andthe like), transmission fluids, cutting fluids, oils, and the like,provided that the dye used is soluble therein. The attenuating mediumcould also comprise very fine dye powder or a gas or a liquid with astrong absorption band at the illumination wavelength.

When the reflectivity of the test part surface is low, a fluorescent dyemay be employed in dye fluid 20 in place of attenuation dye aspreviously described. The fluorescent dye may then be illuminated at asuitable wavelength to cause fluorescence in the visible region. Theintensity of light incident on the camera from each point in the cameraimage will be a function of thickness of the fluorescent dye layer and,therefore, the separation between the test and master surfaces. When afluorescent dye is used, the light observed at the camera is notreflected off the surface but is rather light generated by irradiationof the dye and its resulting fluorescence. Such a system can also beused to gauge deviations from a transparent test part (e.g., glass)where there will be essentially no reflectance.

The deviations from a nominal surface geometry for transparent parts orlow reflectivity parts can also measured by first coating the surface tobe gauged with a reflective coating. For example, the surface of a glasspart could be coated with a thin silver coating. Such a coating could beremoved after the measurements are completed (e.g., a silver coatingcould be removed by an acid wash). As one skilled in the art willrealize, such a coating should be as thin as practical to avoidsignificant loss of resolution which could result from the coating"filling in" or "bridging" depressions and the like in the surface.

An optical system with variable depth resolution can also be obtained byusing a photochromic dye as the attenuating fluid. In such amodification, the sensitivity of the system can be selectively varied byvarying the attenuation of the dye. For example, dye opacity can be setto a desired level by varying the illumination of the dye with a brightlight source. The light source can then be turned off, and using a lowerintensity calibrated illumination system, the reflected image from thetest surface can be quickly measured before dye opacity changessignificantly.

Variable depth resolution can also be obtained by taking multiplemeasurements with attenuating medium containing varying amounts of dye.Generally, improved resolution will be obtained for shallow depressionsor imperfections with a relatively concentrated dye solution. In such acase, the difference between the nominal distance traveled (d_(n)) andthe actual distance traveled (d_(a)) by the light through theattenuating medium is small. By increasing the dye concentration, thedifference in travel (d_(a) -d_(n)) will result in a correspondinglygreater difference in light intensity at the camera. For deepdepressions or imperfections, greater range can be obtained with lessconcentrated dyes. For a surface with both shallow and deep depressionsor imperfections, it may be preferred to obtain measurements atdifferent dye concentrations. The data generated at the different dyeconcentrations can be manipulated and combined by software techniques toobtain a single surface profile geometry or map with variableresolution. Image areas of lower interest can be masked out or capturedat lower resolution while recording critical areas at higher resolution.In this manner, surface profile images with widely ranging resolutionsand details can be generated.

The resolution of the system (especially for the non-depth portion) isdetermined in large part by camera geometry. For example, if a CCDcamera with a 512×512 element array were used to image a surface 50cm×50 cm, each pixel would correspond to about 1 mm² of the surface. Theresolution of the system can be decreased or increased as needed usingvarious techniques. For example, a CCD camera with a larger array couldbe used. If the image array of such a camera was increased to 1024×1024elements, each pixel would correspond to about 0.24 mm² of the same 50cm×50 cm surface (i.e., approximately four fold increase in resolution).Resolution may also be modified by changing the effective focal lengthof the camera lens. By moving camera 34 closer to the test surface 12(i.e., moving the camera in the vertical direction in FIG. 2) willincrease the resolution but will decrease the percentage of the testpart surface that can be observed with a given measurement. To obtainfull analysis or coverage of the test part surface it may be necessary,in such a case, to take multiple measurements for a given part. Suchmultiple measurements could be made by moving the master surface andtest part while holding the camera fixed or, preferable, by moving thecamera into the desired positions (i.e., moving the camera in thehorizontal direction in FIG. 2) using translator 60 (as shown in FIG. 2)to obtain complete coverage of the test part surface. By combining themeasurements, analysis of the entire surface can be obtained. Translator60 can also be used to vary the distance between the camera 34 and thetest part surface 12. If desired, separate translators can be used tocontrol movement of the camera in the vertical and horizontaldirections. Preferably the translator 60 or translators are undercomputer control. The effective focal length and, therefore, cameraresolution can also be modified by use of a zoom-type lens on the camera34. Such a lens would eliminate the need for movement of the camera inthe vertical direction. Again, it is preferred that the zoom-type lensis under computer control. For these general purposes, camera 34 iscoupled to a translator 60, as shown in FIG. 2, which is controlled bycomputer 42. Although not shown, similar techniques and equipment can beused in the modified gauging systems shown in FIGS. 6 and 7.

If it is desired to measure the flatness of the surface of a machinedpart to high tolerances, an optical flat can be used as the masterreference surface. In order to protect the optical surface--which isgenerally relatively expensive--from potential damage caused by placinga machined part in contact with the optical surface, a plurality of thinmasks, shims, or spacers 22 may be placed between the two objects. Theseshims 22 would typically be placed between the optical surface and thesurface to be measured at known fixed points at which surface deviationsdo not need to be measured. Such shims 22 are illustrated in FIG. 2. Insome cases it may not be possible to locate the shims at positions wheresurface deviations do not need to be measured. In such cases, twodifferent measurements can be made with the shims at different positionsto obtain complete coverage of the surface of interest. Alternatively, ajig system that contains mechanical stand-offs or a mechanical fixture108 (see FIG. 1) can be used to hold the part and prevent the object'ssurface from coming into contact with the surface of the optical flat.

Generally, however, shims 22 will be preferred due to their simplicityand their possible use as calibration markers. Such calibration markerscan consist of grooves or slots of known and uniform depth cut ormachined directed into the shims. Or grooves of variable depth can beused where the depth of the groove at various locations along the grooveis precisely known. In addition to providing for general calibration ofthe apparatus, such calibration markers can, as noted above, be used toprovide the necessary calibrations used in eliminating the effect ofvarying reflectance of the surface.

As noted above, optical flats are relatively expensive to prepare andcan be damaged if the optical flat and the test part surfaces come intocontact. Shims 22, as noted above, are one way to minimize damage to theoptical flat used as the master surface. As one skilled in the art willrealize, however, the master surface will eventually be damaged duringuse and the probability of damage will increase as the number of partstested increases. Another way in which to minimize damage to the opticalflat is to simply eliminate its use as the master surface. Rather acommercial-grade glass plate (e.g., float plate glass) can be used asthe master surface and the optical flat can be used as a "standard" testpart to calibrate the glass plate. By placing the "standard" test parton the master surfaces, preferably with shims 22 supporting the"standard" test part, the differences between the glass plate and theoptical flat can be measured and stored in the computer. By measuringactual test parts against the glass plate and using the stored opticalflat calibration data, the actual test parts can be compared to theoptical flat without exposing the optical flat "standard" test part topotential damage. Recalibration using the "standard" test part will benecessary from time to time to simply check the system's operatingcharacteristics or whenever the glass plate master surface is replaced.In any event, exposure of the relatively expensive optical flat testpart to potential damage will be significantly reduced.

Another method for avoiding the preparation and expense of an opticalflat or other master surface, and for avoiding damage to such a mastersystem, is to use the system shown in FIGS. 6 and 7 and discussed abovewherein the attenuating fluid/air interface is used as the reference ormaster surface.

Generally, as noted above, it is preferred that the electromagneticradiation used is in the visible portion of the spectrum. Other forms ofelectromagnetic radiation can be used and in some cases may bepreferred. For example, microwave radiation can be used to gauge bothexterior and interior surfaces of complex metal parts (such as amachined mold or a casting). To use microwave radiation, a gauge blockor die is machined from a dielectric material that is highlytransmissive at the microwave frequency being used. This gauge block ismachined to have exterior and interior surfaces that are nearly amatched surface of the object being measured. The exact dimensions ofthe gauge block are chosen so that the gauge fits closely against (andinside if need be) the surfaces of the object being tested. Thedielectric gauge block is then fitted next to (i.e., placed against orinto as is appropriate) the metal surfaces to be measured. Thedielectric gauge block is irradiated with microwaves that transmitthrough the dielectric and onto the metal surfaces. The strength of thesignal reflected from all points (interior and exterior) on the objectis first measured with a microwave detection system. Next, a partiallyconductive fluid (e.g., a dielectric fluid containing some carbonpowder) is placed between the gauge block and the test object, the gaugeblock and the test object are again fitted together, and the dielectricgauge block is again irradiated with microwaves. The microwaves transmitthrough the dielectric, are attenuated by the resistive dielectricfluid, are reflected back from the metal surface, are attenuated againby the resistive fluid, and finally propagate back out through thedielectric gauge block. The strength of the microwave signals reflectedfrom each point (interior and exterior) on the test object is measuredand digitized. Using the intensity of signal measured at each point onthe test object surface, the signal attenuation can determined. From theattenuation, the separation (thickness of the layer of attenuatingfluid) at all surface points between the gauge block and the test objectis determined. By comparing the measured separation with the designspecifications for the test object, deviations in the shape of amanufactured object from its design specifications are directlymeasured.

I claim:
 1. A system for gauging deviations of a surface on a test partfrom a preselected nominal surface geometry using electromagneticradiation, said system comprising:(1) a container adapted to hold thetest part; (2) an attenuating fluid within the container such that theattenuating fluid substantially covers the test part surface to begauged when the test part is placed within the container and such thatan attenuating fluid/air interface is formed which is suitable for useas a reference surface; (3) a source of electromagnetic radiation forirradiating the test part surface to be gauged; (4) an image sensorpositioned to receive electromagnetic radiation reflected from the testpart surface to be gauged that originated from said source, saidreflected radiation passes back through the attenuating fluid and acrossthe reference surface prior to being received by said image sensor, theintensity of the reflected radiation varying across the image as afunction of the distance between the test part surface and the referencesurface, whereby an image of the test part surface to be gauged isformed; (5) a digitizer for converting the image from the image sensorinto digital signals indicative of the intensity of the reflectedradiation across the image; (6) digital electronic storage coupled tothe digitizer for receiving and storing the digital signals; and (7) acalibration arrangement for correcting errors in the image formed bysaid image sensor; including means for producing a set of correctiondata and for altering the digital signals in accordance with thecorrection data.
 2. A system as defined in claim 1, wherein theattenuating fluid includes a material for attenuating theelectromagnetic radiation as a function of distance the electromagneticradiation travels through the attenuating fluid.
 3. A system as definedin claim 2, wherein the attenuating fluid is a liquid containing a dyeand the electromagnetic radiation is in the visible range.
 4. A systemas defined in claim 2, wherein the attenuating fluid is conductive tomicrowave radiation and the electromagnetic radiation is in themicrowave range.
 5. A system as defined in claim 1, wherein the surfaceof the test part has low reflectivity and the attenuating fluid containsa fluorescent dye responsive to the electromagnetic radiation from theelectromagnetic radiation source.
 6. A system as defined in claim 1,wherein the attenuating fluid contains a photochromic dye havingattenuation characteristics that vary as a function of the intensity ofthe electromagnetic radiation.
 7. A system as defined in claim 1,wherein the image sensor is a CCD sensor having a matrix of imagesensing elements in a row-and-column array, each of the image sensingelements receiving a corresponding location of the image such that theresolution of the system depends upon number, size, and spacing betweenthe image sensing elements.
 8. A system as defined in claim 7, furthercomprising a translator for moving the image sensor relative to thereference surface so as to vary the resolution of the system.
 9. Asystem as defined in claim 1, wherein said calibration arrangement isused to correct for non-uniform reflectivity from the surface of thetest part, and wherein said calibration arrangement comprises:(1)filtering means adapted to be positioned at a preselected point alongthe path defined by the electromagnetic radiation as the radiationtravels from said source to the test part surface and reflects from thetest part surface and travels through said attenuating fluid toward saidimage sensor between said radiation source and said image sensor forvarying the wavelength of radiation collected at said image sensor; andwherein, (2) said storage means stores the digital signals from theimage sensor at a minimum of two different wavelengths; (3) saidproducing means produces the set of correction data based on the digitalsignals stored from the measurements at the different wavelengths; and(4) means for storing the correction data, whereby the correction datacan be used to correct for non-uniform reflectivity from the surface ofthe test part.
 10. A system as defined in claim 1, wherein the containeris equipped with vibration-damping means to allow the attenuating fluidto quickly stabilize.
 11. A system as defined in claim 1, wherein asecond fluid is placed upon the attenuating fluid to increase theresolution of the system where the second fluid floats upon theattenuating fluid.
 12. A system for gauging deviations of a surface on atest part from a preselected nominal surface geometry usingelectromagnetic radiation where the test part surface to be gauged formsa fluid receptacle, said system comprising:(1) an attenuating fluidfilling the fluid receptacle such that an attenuating fluid/airinterface is formed which is suitable for use as a reference surface;(2) a source of electromagnetic radiation for irradiating the test partsurface to be gauged; (3) an image sensor positioned to receiveelectromagnetic radiation reflected from the test part surface to begauged that originated from said source, said reflected radiation passesback through the attenuating fluid and across the reference surfaceprior to being received by said image sensor, the intensity of thereflected radiation varying across the image as a function of thedistance between the test part surface and the reference surface,whereby an image of the test part surface to be gauged is formed; (4) adigitizer for converting the image from the image sensor into digitalsignals indicative of the intensity of the reflected radiation acrossthe image; (5) digital electronic storage coupled to the digitizer forreceiving and storing the digital signals; and (6) a calibrationarrangement for correcting errors in the image formed by said imagesensor; including means for producing a set of correction data and foraltering the digital signals in accordance with the correction data. 13.A system as defined in claim 12, wherein the attenuating fluid includesa material for attenuating the electromagnetic radiation as a functionof distance the electromagnetic radiation travels through theattenuating fluid.
 14. A system as defined in claim 13, wherein theattenuating fluid is a liquid containing a dye and the electromagneticradiation is in the visible range.
 15. A system as defined in claim 13,wherein the attenuating fluid is conductive to microwave radiation andthe electromagnetic radiation is in the microwave range.
 16. A system asdefined in claim 12, wherein the surface of the test part has lowreflectivity and the attenuating fluid contains a fluorescent dyeresponsive to the electromagnetic radiation from the electromagneticradiation source.
 17. A system as defined in claim 12, wherein theattenuating fluid contains a photochromic dye having attenuationcharacteristics that vary as a function of the intensity of theelectromagnetic radiation.
 18. A system as defined in claim 12, whereinsaid calibration arrangement includes filtering means adapted to bepositioned at a preselected point along the path defined by theelectromagnetic radiation as the radiation travels from said source tothe test part surface and reflects from the test part surface andtravels through said attenuating fluid toward said image sensor betweensaid radiation source and said image sensor for varying the wavelengthof radiation collected in said image sensor; and wherein,said storagemeans stores the digital signals from said image sensor at a minimum oftwo wavelengths; and said producing means produces the set of correctiondata based on the digital signals stored from the measurements at thedifferent wavelengths, whereby the correction data can be used tocorrect for non-uniform reflectivity from the test part surface.
 19. Asystem as defined in claim 12, wherein the test part is coupled withvibration-dampening means for facilitating a stabilization of theattenuating fluid within the fluid receptacle.
 20. A system as definedin claim 12, wherein a second fluid is placed upon the attenuating fluidsuch that said second fluid floats upon the attenuating fluid forincreasing the resolution of the system.
 21. A method of gaugingdeviations of a surface on a test object from a preselected nominalsurface geometry using electromagnetic radiation, comprising the stepsof:(A) providing a container substantially filled with an attenuatingfluid; (B) placing the test object within the container such that thesurface of the test object to be gauged is submerged in the attenuatingfluid; (C) irradiating the test object surface with the electromagneticradiation; (D) collecting the electromagnetic radiation that isreflected from the test object surface and passes through theattenuating medium to form an image of the reflected radiation whereinthe intensity of the reflected radiation varies across the image as afunction of the distance between the test object surface and thereference surface; (E) digitizing the image of the reflected radiation;(F) producing a set of correction data related to errors in thedigitized image; and (G) altering the digitized image in accordance withthe correction data.
 22. The method of claim 21, wherein steps (F) and(G) are performed by the substeps of:positioning a filter in the path ofthe electromagnetic radiation in order to vary the wavelength of theradiation collected in step (D); storing the digital signals collectedat a minimum of two wavelengths; producing the correction data using thedigital signals stored from the different wavelength measurements; andusing the correction data to correct for non-uniform reflectivity fromthe test object surface.
 23. The method of claim 21 wherein steps (F)and (G) are performed by the substeps of:measuring and storing theintensity of the radiation reflected from the test surface across theimage with attenuating fluid having a first concentration of dyepresent; and using the measured intensity across the image to correct ameasurement made with the attenuating fluid having a secondconcentration of dye present.
 24. The method of claim 21, furthercomprising the step of placing a second fluid upon the attenuating fluidsuch that the second fluid floats upon the attenuating fluid, prior toirradiating the test object surface, to thereby increase the resolutionof the image formed.
 25. A method of gauging deviations of a surface ona test object from a preselected nominal surface geometry where the testobject surface to be gauged forms a fluid receptacle, comprising thesteps of:(A) filling the fluid receptacle with a preselected attenuatingfluid; (B) allowing the attenuating fluid to settle such that theinterface between the attenuating fluid and the air adjacent theattenuating fluid is suitable for use as a reference surface; (C)irradiating the test object surface with electromagnetic radiation; (D)collecting the radiation reflected from the test object surface andattenuated through the attenuating fluid to form an image of thereflected radiation wherein the intensity of the reflected radiationvaries across the image as a function of the distance between the testobject surface and the reference surface and wherein there may existoptically generated errors in said image; (E) digitizing the image ofthe collected radiation; (F) producing a set of correction data relatedto errors in the digitized image; and (G) altering the digitized imagein accordance with the correction data.
 26. The method of claim 25,wherein steps (F) and (G) are performed by the substeps of:positioning afilter in the path of the electromagnetic radiation in order to vary thewavelength of the radiation collected in step (D); storing the digitalsignal collected at a minimum of two wavelengths; producing calibrationsignals based on the digital signals stored from the differentwavelength measurements; and using the calibration signals to correctfor non-uniform reflectivity from the test object surface.
 27. Themethod of claim 25, wherein steps (F) and (G) are performed by thesubsteps of:measuring and storing the intensity of the radiationreflected from the test object surface across the image with anattenuating fluid having a first concentration of dye present; and usingthe measured intensity across the image to correct a measurement madewith the attenuating fluid having a second concentration of dye present.28. The method of claim 25, further comprising the step of placing alayer of a preselected second fluid on the attenuating fluid such thatthe second fluid floats upon the attenuating fluid to thereby increasethe resolution of the image.
 29. The method of claim 25, furthercomprising the steps of coupling the test object to a vibration dampingmeans and settling the attenuating fluid, using the vibration dampingmeans, prior to performing step (C).
 30. A system for gauging deviationsof a surface on test part from a preselected nominal surface geometryusing electromagnetic radiation, said system comprising:a containeradapted to hold the test part; an attenuating fluid within the containersuch that the attenuating fluid substantially covers the test partsurface to be gauged when the test part is placed within the containersuch that an attenuating fluid/air interface is formed which is suitablefor use as a reference surface; a source of electromagnetic radiationfor irradiating the test part surface to be gauged; an image sensorpositioned to receive electromagnetic radiation reflected from the testpart surface to be gauged that originated from said source, saidreflective radiation passes back through the attenuating fluid andacross the reference surface prior to being received by said imagesensor, the intensity of the reflected radiation varying across theimage as a function of the distance between the test part surface andthe reference surface, whereby an image of the test part surface to begauged is formed; a digitizer for converting the image from the imagesensor into digital signals indicative of the intensity of the reflectedradiation across the image; digital electronic storage coupled with thedigitizer for receiving and storing the digital signals; a calibrationarrangement for correcting errors in the image formed by said imagesensor; including means for producing a set of correction data and foraltering the digital signals in accordance with the correction data; anda second fluid placed upon the attenuating fluid to increase theresolution of the system where the second fluid floats upon theattenuating fluid.
 31. A method of gauging deviations of a surface upona test object from a preselected nominal surface geometry usingelectromagnetic radiation, comprising the steps of:(a) providing acontainer substantially filled with an attenuating fluid; (b) placingthe test object within the container such that the surface of the testobject to be gauged is submerged in the attenuating fluid; (c)irradiating the test object surface with the electromagnetic radiation;(d) collecting the electromagnetic radiation that is reflected from thetest object surface and passes through the attenuating fluid to form animage of the reflected radiation wherein the intensity of the reflectedradiation varies across the image as a function of the distance betweenthe test object surface and the reference surface; (e) digitizing theimage of the reflected radiation; (f) producing a set of correction datarelated to errors in the digitized image; (g) altering the digitizedimage in accordance with the correction data; and (h) placing a secondfluid upon the attenuating fluid such that the second fluid floats uponthe attenuating fluid, prior to irradiating the test object surface, tothereby increase the resolution of the image formed.
 32. A method ofgauging the deviations of a surface on a test object from a preselectednominal surface geometry where the test object surface to be gaugedforms a fluid receptacle, comprising the steps of:(a) filling the fluidreceptacle with a preselected attenuating fluid; (b) allowing theattenuating fluid to settle such that the interface between theattenuating fluid and the air adjacent the attenuating fluid is suitablefor use as a reference surface; (c) irradiating the test object surfacewith electromagnetic radiation; (d) collecting the radiation reflectedfrom the test object surface and attenuated through the attenuatingfluid to form an image of the reflected radiation wherein the intensityof the reflected radiation varies across the image as a function of thedistance between the test object surface and the reference surface andwherein there may exist optically generated errors in said image; (e)digitizing the image of the collected radiation; (f) producing a set ofcorrection data related to errors in the digitized image; (g) alteringthe digitized image in accordance with the correction data; and (h)placing a layer of a preselected second fluid on the attenuating fluidsuch that the second fluid floats upon the attenuating fluid to therebyincrease the resolution of the image.